1. Field of the Invention
This invention relates to photography, and more particularly, it relates to photographic processes performed in ambient light and to photographic products useful in such processes wherein the compounds employed as optical filter agents for protecting the imagewise exposed photosensitive material from further exposure during processing decolorize independently of changes in pH.
2. Description of the Prior Art
A number of diffusion transfer processes for producing photographic images in both black-and-white and in color have been proposed. Of particular interest are diffusion transfer processes wherein the image-receiving layer carrying the transfer image is not separated from the developed photosensitive layer(s) after processing but both components are retained together as a permanent laminate. Included as part of the laminate is a layer of light-reflecting material, preferably titanium dioxide, positioned between the image-carrying layer and the developed photosensitive layer(s). The light-reflecting layer separating the image-carrying and photosensitive components provides a white background for the transfer image and masks the developed photosensitive layer(s). In addition to these layers, the laminate usually includes dimensionally stable outer layers or supports, at least one of which is transparent so that the resulting transfer image may be viewed by reflection against the background provided by the light-reflecting layer. Diffusion transfer processes for forming images viewable without separation of the photosensitive and image-receiving components and integral negative-positive film units useful in such processes, i.e., film units wherein the negative or the photosensitive component and the positive or image-receiving component are retained as a permanent laminate after processing are described in U.S. Pat. Nos. 3,415,644, 3,415,645, and 3,415,646 issued Dec. 10, 1968 to Edwin H. Land, U.S. Pat. Nos. 3,573,043 and 3,573,044 issued Mar. 30, 1971 to Edwin H. Land and U.S. Pat. Nos. 3,594,164 and 3,594,165 issued July 20, 1971 to Howard G. Rogers.
U.S. Pat. No. 3,647,437 issued Mar. 7, 1972 to Edwin H. Land also is concerned with diffusion transfer processes wherein the resulting photograph comprises the developed photosensitive layer(s) retained with the image-receiving layer as part of a permanent laminate. In the processes disclosed in this patent, a photographic film unit comprising a photosensitive element is developed in ambient light but further undesired exposure during processing is prevented by a light-absorbing material or optical filter agent which is retained in the processed film unit. In a preferred embodiment, the optical filter agent is a pH-sensitive dye, i.e., a dye possessing sepctral absorption characteristics that are reversibly alterable in response to changes in environmental pH and particularly, a pH-sensitive dye having a colored or light-absorbing form above a given alkaline pH and a colorless or non-light-absorbing form below said pH. Though the pH-sensitive dye is usually included in the processing composition, it may be initially positioned in the film unit, for example, in a layer over the photosensitive element provided it is in its colorless form if photoexposure is to be effected through that layer. Upon application of an alkaline processing composition, the pH-sensitive dye is converted to its colored form, and after the desired processing time, it is converted back to its colorless form by reducing the environmental pH, e.g., by including an acid-reacting layer as part of the film unit. Examples of pH-sensitive dyes found particularly useful as optical filter agents are the phthalein and naphthalein dyes disclosed in U.S. Pat. No. 3,702,244 issued Nov. 7, 1972 to Stanley M. Bloom, Alan L. Borror, Paul S. Huyffer and Paul T. MacGregor and in U.S. Pat. No. 3,702,245 issued Nov. 7, 1972 to Myron S. Simon and David P. Waller and also the 9-pyridyl fluorene dyes disclosed in U.S. Pat. No. 3,726,675 issued Apr. 10, 1973 to Alan L. Borror.
The present invention is directed to a different class of optical filter agents and filter agent precursors useful in the above-described processes and products wherein a colored optical filter agent formed by contacting the precursor with a basic processing composition is decolorized without reversal to the precursor and without reducing the pH. Because of their ability to clear independently of a pH reduction, the subject filter agents may be employed in photographic processes where the pH of the system remains substantially unchanged subsequent to processing and also may be employed to permit early viewing of the final image in processes where the pH is reduced during the final stages of processing. Also, because the optical filter agent is irreversibly decolorized to a colorless product inert to changes in pH, the possibility of color reappearing in time due to accidentally increasing the pH is avoided.